1. Field of Invention
This invention relates to the scrambling of analog (particularly voice) signals for secure transmission to authorized receivers.
2. Description of Prior Art
In recent years, a good deal of research has been directed toward providing means whereby voice and other audio signals may be transmitted to remote users in such manner that unauthorized listeners are unable to decipher the message, if they are able to receive it at all. The military, in particular, employ highly sophisticated signal transmission methods utilizing statistical and other complex techniques. Apparatus embodying these techniques are ordinarily very expensive. However, the need for complete security in the applications in which they are utilized, easily justifies their cost.
There are, however, other users of communication apparatus, such as police agencies and the financial community, which also have a need for secure transmission of messages. In most cases, however, these users do not have a need for such highly sophisticated and expensive apparatus, since the messages they ordinarily wish to transmit are not of the same order of strategic importance, and their "adversaries" are generally less sophisticated. The systems presently available to such users provide only modest security and are too expensive to enable their widespread use, for example, in police squad cars.